This invention relates to a control apparatus for an automatic transmission in an automobile or the like, of the type having power transmitted from the engine to a speed-change gear mechanism through a fluid type torque converter, and a separate frictional engagement device disposed in parallel to the torque converter between the engine and the speed-change gear mechanism. More particularly, this invention relates to the aforesaid type of control apparatus for an automatic transmission which automatically and hydraulically controls the operation of the speed-change gear mechanism and the direct-linkage frictional engagement device in response to the engine output and vehicle speed.
Conventional automatic transmissions include types where power from an engine is transmitted to a speed-change gear mechanism through a fluid type torque converter. The damping effect of the torque converter provides vehicle speed changes having reduced shock sensations, without the need to intercept the transmitting of engine power as by a clutch device. Additionally, acceleration can be maximized during a low speed drive condition by use of the torque-increasing action. However, slippage is always occurring among the pump impeller, turbine runner and stator of a conventional torque converter in operation of an automatic transmission. The power transmission efficiency is therefore lower than in automatic transmissions having mechanical clutch mechanisms and results in an increase in fuel consumption and a reduction in the engine braking effect.
To overcome these disadvantages in automatic transmissions utilizing torque converters, it has been proposed to provide a frictional engagement device such as a clutch mechanism of the direct-linkage type in parallel to the torque converter. According to this proposal, power is transmitted through the torque converter only when the function of the torque converter is required, as for example, when the vehicle speed is changed or the torque is increased. At other times, primarily during a relatively high vehicle speed and constant drive condition, the power is transmitted at a high efficiency by means of the direct-linkage clutch mechanism.
If a manual system were used for controlling the torque converter and the direct-linkage clutch mechanism to achieve the above-described purpose, the operation becomes complicated and attainment of the desired control would vary depending on the skill of the driver. This would not be a practical system. Alternatively, if an electrical system were used to achieve this control, an electric control circuit would have to be provided, in addition to a fluid-pressure, speed-control system, for actuation of the speed-change mechanism of the automatic transmission using additional clutches or brakes. Accordingly, the total structure required becomes complicated and the reliability is not high.